Recycling of cellulosic paper is a major activity in the papermaking industry. In certain waste paper, such as “office waste”, a major problem associated with the recycling process is the separation of paper fibers or small clumps of paper fibers from ink particles which have been dislodged from the recycled paper. Office papers or “office waste” are terms used at times to designate paper waste which originates in business offices or the like. Office waste often includes relatively large quantities of ink from sources such as toners and the like employed in copying machines, computer printers, and other means by which ink is physically applied onto cellulosic paper.
Commonly the ink is in the form of black or other colored particulates and once applied to a paper, the particulates are strongly bonded (at times “fused”) to the paper fibers. Thus, the initial effort toward recycling a printed paper is to dislodge the ink particulates from the paper fibers. In paper recycling activities, this is commonly accomplished by disintegrating the printed paper into very small pieces which are either simultaneously or subsequently converted to a pulp which contains a pulping liquid, fibers and small bunches of fibers, and dislodged ink particles. The dislodged ink particles may be larger or smaller than individual ones of the paper fibers.
In some cases, an agglomerating chemical is added to the pulp to consolidate smaller ink particles into larger ink particles. The pulp, so treated, is thereafter subjected to screening wherein the fibers, and smaller ink particles are passed through the screen along with the carrier liquid. The filtrate from the screening, containing fibers and small ink particulates is thereafter subjected to treatment employing a cyclone cleaner wherein small ink particulates are separated from the fibers to develop an output stream of carrier liquid and fibers, some of which retain ink particles attached thereto. This output stream contains a slurry of fibers and small ink particles.
Rejects from the screen include larger fiber bundles, various contaminants such as plastic, ink particles which do not pass through the screen openings, and ink remaining associated with the fibers in the fiber bundles.
U.S. Pat. No. 5,512,133, the entire content of which is incorporated herein by reference, provides a description of the deinking art, including a definition of “ink”, sources of ink-bearing recycled paper, prior art methodology for deinking the paper, and a deinking method which employs agglomeration agents, screening, cyclone cleaning and flotation steps.
U.S. Pat. No. 3,635,788, the entire content of which is incorporated herein by reference, provides a description of the use of cyclone centrifuges in the deinking of recycled paper stock.
From the foregoing it will be recognized that conventional deinking systems use many stages of pulp treatment to improve the quality and cleanliness of the pulp, including stages using equipment for screening, centrifugal cleaning, flotation, washing, pressing, dispersion, and bleaching. In the instance of the flotation stage, pre-dispersion of the ink in a high consistency disperser is usually required to thoroughly detach the ink from the fiber and to break it down into fine specks which can be successfully removed by the flotation stage. The dispersion operation involves thickening the pulp to 25-35% consistency, then heating it with steam and then dispersing it in a refiner type device. The pulp consistency is then reduced to about 1% for ink removal in the flotation stage.
Dispersion followed by flotation has a high capital cost for the large equipment as well as a large space requirement in the deinking plant. It also involves high operation cost because of substantial fiber loss in the thickening and flotation stages, high usage of horsepower and steam and equipment maintenance expenses. Nevertheless, dispersion/flotation is commonly used because it is very effective for improving the cleanliness of the deinked pulp. Most modern deinking systems operating on office waste actually use dispersion/flotation at two separate points in the deinking system to achieve adequate pulp cleanliness. Clearly, this duplicate arrangement has a particularly high capital cost and a high operation cost.
Dispersion followed by flotation can also be added to an agglomeration deinking process to improve the ink speck removal. It thus becomes evident that there is needed an alternative method for deinking recycled fibrous paper using the agglomeration system without using the dispersion/flotation concepts.